History student here. The USSR's notion of national security after WW2 was in the form of territorial security, which is why they established many satellite states (i.e. states friendly towards USSR, mostly if not all communist) around them. The Marshall Plan would open up the countries' economies to foreign influences, increasing dependency on the global market, was thus viewed as an attempt by the West to disrupt the USSR's sphere of influence over Eastern Europe. The plan was also ideologically incompatible with the USSR's centrally planned economy (which it was also imposing on its satellites).

Any attempt to revive Germany was also considered a threat by the USSR, given the history of land wars between Russia and Germany even before WW1 and 2, and any attempt led by the capitalist Western nations naturally led to some serious suspicion on the part of USSR. A weak Germany that they could milk reparations from was the preferred option.

Also I should add that the Marshall Plan was hardly in any way going to be a conduit for communism, as it encouraged the formation of free market and trade, while the USSR advocated a centrally planned economy. In fact, the Marshall plan was a political move in the form of a economic policy, with the intent to contain communism (Truman Doctrine, "containment policy"). It was designed to make communism less appealing upon the realization that communism thrives in poverty, the state most of post-war Europe was in. Stalin was receptive of the idea of Marshall plan initially, but realized that he had more to lose by partaking in it, thus whatever that followed.